One such vehicle brake system is for example known from DE 10 2013 206 324 A1. Said known vehicle brake system consists of an operating brake in which the brake pressure is produced by external force operation of a pressure generator in combination with muscle power operation of a master brake cylinder by the driver and a muscle power operated auxiliary brake that is activated in the event of a malfunction of the operating brake. The pressure generator is directly connected to a reservoir tank for a pressure medium and as a result is able to boost the brake pressure that is set by the driver. As a result, the known vehicle brake system is especially suitable for vehicles that are powered by an electric motor, a hybrid drive or by a supercharged internal combustion engine, and with which owing to its drive concept no vacuum, or in any case only a small vacuum, can be tapped that could be used for conventional brake force boosting by means of a pneumatic brake force booster. The known vehicle brake system comprises a plurality of brake circuits. A pressure generator and moreover at least one wheel brake are associated with each brake circuit. A pressure modulation device is connected between the pressure generator and each wheel brake, which adapts the pressure in the wheel brake to the slip characteristics at the associated wheel. For this purpose, the pressure modulation device comprises a regulated pressure build-up valve disposed in the brake circuit upstream of the wheel brake and a pressure reducing valve disposed downstream of the wheel brake. Said pressure reducing valves are designed in the form of switching valves that are closed in the normal position. Switching valves are available on the market relatively inexpensively.
With the known vehicle brake system, the driver is always involved in the build-up of brake pressure during operating brake processes and is consequently hydraulically connected to the to wheel brake. Said hydraulic connection causes a loss of comfort, as pressure pulsations in the brake circuit that are triggered by the pressure generator can be detected within the interior of the vehicle by means of the brake pedal that is coupled to the master brake cylinder.
In vehicles with a pure electric motor drive or with a hybrid drive, in which an electric motor and an internal combustion engine are used together as drive sources, so-called regenerative braking processes can be carried out. Said regenerative braking processes include braking processes during which energy recovery is carried out by converting the kinetic energy of the vehicle into electrical energy, which can then be stored in an accumulator. The total braking effect of a corresponding vehicle during said braking processes is comprised of a hydraulic braking torque of the conventional hydraulic brake system and a regenerative brake torque of the electrical drive operated as a generator. By blending the two braking torques, the total braking torque during a braking process can be held constant. However, the regenerative braking depends on the speed of the vehicle. Disadvantageously, owing to the nature of the system the known vehicle brake system is therefore not capable of fully blending the regenerative brake torque with the hydraulic brake torque until the vehicle is at a standstill. This adversely affects the efficiency of the energy recovery.
Fully blending capable, power-assisted brake systems with an electric motor driven plunger as the pressure generator are currently in the development stage. By reversing the direction of motion of said plunger, the wheel brake pressure can be reduced relatively simply and without the involvement of other components, for example of solenoid valves. It is disadvantageous, however, that a plunger only has a structurally limited stroke available to it, or that at the end of said stroke travel a reversal of the direction of motion of the plunger is necessary to maintain the operability thereof. Additional plunger control valves are necessary in this case in order to avoid pressure medium from being sucked out of the wheel brakes as a result of the restoring displacement of the plunger and consequently inadvertently reducing the brake pressure. Plunger control valves are still in the development stage, increase the parts costs and the assembly costs of a hydraulic assembly of such a vehicle brake system, occupy additional installation space and cause a bulky structure of the hydraulic assemblies and a correspondingly high use of raw material for the hydraulic block of the hydraulic assembly.